This invention relates to foam and flame-breakable resin compositions.
In recent years, there has been increasing demand for materials having superior non-inflammability, usable for example, in insulated wire and cable, buildings and structures, machines, tools, etc. Plastic materials having such properties have been increasingly used for insulated wire and cable. More stringent requirements have been established for those plastic materials, for example as set forth in UL (Underwriter's Laboratories, Inc) Standards, especially for consumer items.
However, for insulation for high voltage circuits and high frequency circuits, etc, inevitably, a polyethylene which is a very inflammable polymer has been used to insulate wire and cable. Disadvantageously, such polyethylene tends to burn easily in a fire emergency.
In order to decrease inflammability of polyethylene, attempts have been made to make the polyethylene insulator itself noninflammable. Also, a sheathing layer (used about the polyethylene layer) having an ability to prevent flames from touching the polyethylene, has been used.
A superior non-inflammable sheathing material for wire and cable is needed. As a flame test method to examine the non-inflammability of such wire and cable, IEEE Std. 383-1974 is the standardixed test usually used. Since the sheating material of wire and cable should have a superior non-inflammability of high degree in order to pass such stringent tests, a material which has both properties as a material for cable sheating and superior non-inflammability of high degree meeting the flame test standard, has not yet been realized. In order to make a cable having a desired foam and flame breakable property, previously, the cable was coated with a special material having superior flame proof property after installation of the cable. The sheathing layer covers the inner insulating layer of polyethylene. The sheathing material protects the inner polyethylene layer from flames by retaining its state as a sheathing layer when subjected to flames and gradually foaming but not dropping off by fusion. In this manner the inner easily inflammable layer of polyethylene is not exposed to flames. Moreover, even if the sheathing layer is carbonized or burnt to ash, it should retain a protective layer to prevent the inner, easily inflammable insulating layer from directly contacting the flame.
Previously, as a non-inflammable sheathing material for insulated wire and cable, polyvinyl chloride (hereinafter called "PVC") has been mainly used and on other occasions, various copolymers containing ethylene, vinyl acetate, and vinyl chloride have been used.
As is well known, and although PVC itself has good non-inflammability property, the flame breakable ability of PVC is substantially decreased by the presence of plasticizers. For this reason, PVC compounds cannot be a superior flame breakable material. It is often necessary to add a large amount of plasticizer, such as more than 40 parts per 100 parts of PVC (in terms of weight), in order to impart necessary degree of workability, flexibility and softness at a low temperature. This type of soft PVC is used in the sheathing.
Even though PVC is a material having high oxygen index and has good non-inflammability, since soft PVC composition contains a large amount of plasticizer, its melt viscosity decreases considerably when it is exposed to a flame and accordingly decreases its ability to foam even if a sufficient amount of foaming agent is contained, and then drops off, thus exposing the inner easily inflammable insulating layer to the flame.
Since graft copolymer of ethylene-vinyl acetate-vinyl chloride can be prepared to be soft plastic material without mixing in any plasticizer, the graft copolymer does not drop off by fusion when contacted with a flame, and shows good foamability. However, this material is inferior to PVC resin composition as a sheathing material from the standpoint of oil resistance and heat deformation.